Organic Light Emitting Diodes (OLEDs) are based on inclusion, between the anode and cathode, of an electroluminescent layer formed by a film of semiconductors and luminescent organic compounds that react to a particular electrical stimulation, thus obtaining a device that transforms electrical energy into light energy. Despite its relatively recent creation, OLED technology has developed rapidly due to its potential applications in the manufacture of flat screens, signs, and solid-state lighting elements, among others.
The basic structure of an OLED consists of one or more layers of organic semiconductor material lying between two electrodes (active region). The negative electrode (cathode) is formed by a metal or a metal alloy. The positive electrode (anode) is an optically transparent conductive material (usually glass coated with a transparent conductive oxide such as, for example, tin oxide doped with Indium), so that the light generated in the active region can pass through the anode. The active region in an OLED is typically formed by a layer of a luminescent organic molecule and an organic hole transport layer (HTL). Generally speaking, when a potential difference is applied to the device, the cathode injects electrons to the organic molecule, while the anode extracts electrons, i.e., injects positive charges (holes) in the molecule. In the organic molecule the electron-hole pair corresponding to an excited state can interact to form an exciton, which can decay radiatively generating a photon of energy equal to the difference between the HOMO (High Occupied Molecular Orbital) levels and LUMO (Low Unoccupied Molecular Orbital) of the molecule. In this way, the so-called exciton radiation of the molecule is generated. By Einstein-Planck relation the energy of each photon corresponds to the wavelength (color) of the emitted radiation.
Specifically, an MDMO-PPV/OLED (manufactured from the luminescent polymer MDMO-PPV) has the structure ITO/PEDOT:PSS/MDMO-PPV/METAL shown in FIG. 1. A thin layer of tin oxide doped with Indium (ITO) is used as the anode (A) which is a p-type degenerated semiconductor highly transparent in the visible range that, within the structure of the OLED, operates as a contact hole injector supplying positive charges to luminescent polymer (B) based on the difference between the maximum of the valence band (VBM) of the ITO and the energy of the highest occupied molecular orbital (HOMO) of the luminescent polymer, and the emission of excitons is produced by the organic MDMO-PPV layer. In order to increase the probability of hole injection into the MDMO-PPV layer, a transparent, organic and conductive layer (D) is introduced, made of poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate), abbreviated as PEDOT:PSS, which facilitates transportation of holes from the ITO to the luminescent polymer (B), because it has an intermediate level of energy between the ITO valence band and the polymer HOMO. Hence the PEDOT:PSS acts within the structure as the HTL. Different metals may be used as a cathode (C) that inject electrons to the electroluminescent layer to radiatively recombine with the holes injected from the ITO (anode). The efficiency of electron injection is strongly influenced by the work function of the metal used as the cathode, which is usually silver or aluminum [Mendez, et al., Rev. Col. Fis., 2010, 42, 397-401].
Moreover, within the field of organic electronics, improving the quantum efficiency of OLED emissions is a goal that currently exhibits great scientific activity, because the many potential technological applications (flat screens , lamps, televisions, organic lasers, etc.) of OLEDs depend fundamentally on the ability to develop methodologies that increase this external quantum efficiency. However, there is a great inherent loss in the radiation emitted by the active element in the OLED (MDMO-PPV) design, due to re-absorption of radiation by the device's constituent materials and also to having little control in directing light within the different layers and in particular the interfaces.
The radiation emitted by the active region (MDMO-PPV), can be classified into three main modes (FIG. 1): The waveguide mode, wherein the radiation is trapped in the region formed by the layers of METAL/MDMO-PPV/PEDOT:PSS/ITO due mainly to internal total reflection phenomena in the ITO/substrate and METAL/MDMO-PPV interface. The substrate mode, when the radiation is trapped within the substrate by the ITO/substrate and substrate/air interfaces. The air mode, when the radiation emitted by the active region exits the device into the air. From the viewpoint of classical optics, because of refraction-reflection processes between the different layers constituting the PPV-OLEDs, the device's light extraction efficiency is approximately only 20%, taking into account that the extraction efficiency of OLED light is defined as the ratio of the intensity of radiation in the air mode and the intensity of the total radiation emitted by the active element in the device [F. Masayuki et al., Japanese Journal of Applied Physics. 2005: 44, 3669-3677]. Thus, a number of techniques have been proposed recently that aim to solve the problem of radiation re-absorption in OLEDs. One of the simplest methods to extract some waveguide modes from the substrate is the use of a roughened surface by applying sandblasting on one side of the glass substrate and manufacturing the OLED on the other side. Due to the roughness of guided wave mode substrate in the glass-air boundary, these are coupled outside in the air gap, and the coupling efficiency increases with the roughness of the substrate. Moreover, the extraction of light from OLED using photonic crystals (PhC) arranged in different ways within the OLED structure has been proposed. [K. Saxena et al., Optical Materials, 2009, 32, 221-233]
Thus, the patent application WO2007141364 discloses a process for preparing thin films of colloidal crystal comprising the steps of: a) preparing a colloidal suspension containing the compound particles to deposit as a thin sheet of colloidal crystal, by dispersing said particles in a volatilizable liquid medium during the spin deposition process (spin-coating) and stirring said suspension for a period between 5 minutes and 24 hours, b) applying the colloidal suspension obtained in the previous step on a substrate, previously treated or not, in sufficient quantity to cover said substrate, c) rotating the substrate (spin-coating) with the compound applied in the previous stage at speeds between 1 and 200 revolutions per second for a period between 1 second and 1200 seconds.
Various patent documents illustrate techniques for the development of OLED devices, for example, Patent No. WO2006110926 refers to an OLED device using a polymer emissive layer (MEH-PPV) located between two semitransparent electrodes, where at least one electrode is perforated and the organic semiconductor polymer is a soluble derivative Poly[p-phenylene-vinylene] (PPV).
U.S. Patent document No. U.S. Pat. No. 6,403,238 discloses a process for manufacturing an OLED comprising one or more light-emitting active layers, located between two coated injector contact layers on a substrate, where at least one of the active layers consists of Poly[2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene-vinylene-1,2-ethenylene-2,5-dymethoxy-1,4-phenylene-1,2-ethylene] (M3EH-PPV), where the M3EH-PPV can be optionally mixed with other electrically active materials and applied as a solution film on the substrate.
Colombian Application No. CO6470853 relates to a composite comprising at least two components, wherein at least one component is present in the form of nanoparticles, consisting of at least three metals and at least one non-metal whose diameter is less than one micrometer, preferably less than 200 nm. The composite according to the invention is particularly well suited for the production of photoactive layers.
Document U.S. Pat. No. 8,329,505 discloses a method for the placement of the cathode structure of an OLED diode. The invention comprises a number of potential advantages such as lower device manufacturing time, less material consumption and less equipment.
Patent document EP1929533 relates to a method of manufacturing an OLED display having a plurality of OLED devices. The method includes providing a plurality of OLED devices on a substrate and sharing a common light transmitting electrode, thus forming a conductive layer structure patterned over the common light emitting electrode.
Korean Patent No.KR100873517 refers to an OLED-PhC device and its manufacturing method. The patent discloses a device and a method that improves the quantum efficiency of the OLED by means of a PhC layer. However, the PhC type is not specified. It could be a 2D-PhC by lithography or colloidal crystal. Nor is the polymer used specified. The present invention proposes a method of manufacturing the active region from a single layer consisting of a colloidal crystal (SiO2 spheres 250 nm in diameter with fcc structure) and the luminescent polymer (MDMO-PPV).
Japanese Patent No. JP4533041 proposes improving the quantum efficiency of an OLED by a chemical treatment on the substrate to make it porous, without specifying the pore size or type of substrate.
Korean Patent Application No. KR20030026450 discloses an organic light emitting device that improves quantum efficiency in an OLED by adding a PhC stratum in the upper layer of the device (concave-convex structure). This document, however, does not specify what type of PhC is used.
Patent No. CN 101000949 proposes a method for improving the mono-chromaticity of OLEDs using a colloidal crystal layer. In the method, a layer of (unspecified) luminescent polymer is deposited on a layer of SiO2 spheres without specifying the method by which this layer is deposited. The present invention proposes synthesizing these two layers in a single procedure using a deposition by centrifugation (spin-coating) type method.
Chinese Patent No. CN101409331 relates to an electroluminescent device that improves light extraction by placing a photonic crystal (not specified) on top of the structure. The improved display device can be manufactured using a thermal transfer donor film layer for adhering the photonic crystal to the structure.
Patent application No. US20080284320 proposes a method for improving the quantum efficiency of OLEDs by using a substrate with a photonic crystal, said crystal comprising a film structure on a substrate produced using a combination of materials with high and low refractive indexes.
Patent document No. US2010148158 refers to improving the quantum efficiency of OLEDs having excellent solubility and thermal stability by incorporating a layer of SiO2 in nanopowder by drip coating.
Similarly, various research papers work on optimizing electroluminescent devices. For example Wang B. et al. deals with improving the quantum efficiency of OLEDs using nanosphere lithography (Journal of Crystal Growth, Volume 288, Issue 1, 2 February 2006, Pages 119-122). Likewise, Kim M. et al., in their article entitled Enhanced performance of organic light-emitting diodes using two-dimensional zinc sulfide photonic crystals, refer to improving the quantum efficiency of OLEDs by ZnS PhC grown on the glass substrate (Journal of Applied Physics, Volume: 106, Issue: 11).
Puzzo D. et al. reports a process for improving the quantum efficiency of OLEDs using 1D-PhC of antimony doped TIN (Nano Lett., 2011, 11 (4), pgs. 1457-1462). Likewise, Quang-Cherng H. relates to the manufacture of a photonic crystal structure using nano-printing, which substantially improves the quantum efficiency of PMMA-OLED using 2D-PhC by lithography (Microelectronic Engineering, Volume 91, March 2012, Pages 178-184).
As can be seen, even though incorporating photonic crystals into the OLED structure has proven a viable option for solving the problem of resorption, specific characteristics are still unclear in terms of size, material, and structure of the crystal, or in which part of the structure they should be placed for optimum efficiency of the electroluminescent structure.